A great deal of effort is being devoted to the development of ceramic heat exchangers using the general assembly method of alternately stacking or winding relatively thin flat sheets with corrugated, ribbed or otherwise profiled plates and then subjecting this unfired or green structure to a high firing temperature for hardening. With such heat exchangers, it is desirable to minimize wall thickness to permit greater heat exchanger effectiveness and better use of materials. However, when this is done handling of the relatively weak, green pieces becomes a major problem. Specifically, the green structure tends to sag or deform prior to the firing and hardening procedure which results in undesirable non-uniform passages therethrough.
Ceramic heat exchangers are currently being made by utilizing reaction bonding or sintering techniques to harden a mixture of ceramic powders and binding ingredients. Unfortunately with these techniques the binding ingredients burn out during the initial heating stages leaving open pores which later must be sealed to produce a heat exchanger without internal leakage. This is an expensive processing step, and, therefore, various attempts are being made to increase the final density of the fired ceramic walls to avoid such a porous structure. For example, the density of the thin walls could be increased by subjecting the stacked sheets and profiled plates to pressure prior to the final firing stage. However, such pressure tends to further distort the alternate layers and block the passages therethrough as set forth above. To avoid this, and as an alternate to firing the green ceramic stack of alternate sheets and plates into a monolithic block, the sheets and plates may be individually fired and hardened and thereafter be stacked and bonded together. But here also an entirely new set of complex problems are created, involving the effective bonding of the parts together to avoid cracking and leakage of the heat exchanger with extended service.
Exemplifying the improvements being made in the ceramic heat exchanger field are the following U.S. Pat. Nos.:
2,552,937 to H. Cohen PA1 3,081,822 to J. Wolansky et al. PA1 3,112,184 to R. Z. Hollenbach PA1 3,444,925 to J. R. Johnson
One of the ceramic materials being actively considered for such heat exchangers is silicon nitride. Some of the properties and advantages of this material are set forth in British Patent No. 970,639 to G. G. Deeley, published Sept. 23, 1964; British Patent No. 1,092,637 to R. F. Coe, published Nov. 29, 1967; and British Patent No. 1,266,506 to E. R. W. May, published Mar. 8, 1972.